Problem: Before the previous stop there were 99 people riding on a train. 60 people ended up getting off at the stop. How many people are riding the train now?
Answer: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $99 - 60$ people on the train. $99 - 60 = 39$ people are on the train.